Diphenyl carbonate (DPC) is a less-toxic and contamination-free organic substance and an intermediate of important engineering plastic materials, and can be used in the synthesis of many important organic compounds and macromolecule materials, such as monoisocyanates, diisocyanates, polycarbonates, methyl parahydroxybenzoates, poly(aryl carbonates), etc., and as a plasticizer and solvent for polyamides and polyesters.
Recently, with the development in non-phosgene synthesis of high quality polycarbonate (PC) by using diphenyl carbonate and bisphenol A as the raw materials, diphenyl carbonate becomes a most attention-getting compound. The production of polycarbonate mostly adopts the method of polymerizing bisphenol A with phosgene or methyl chloroformate in a two-phase system of methylene chloride-water (also known as Interfacial Polymerization); however, phosgene and methyl chloroformate are very toxic substances, which will contaminate the environment severely and corrode the equipment. On the contrary, the synthesis of polycarbonate from diphenyl carbonate can avoid using toxic solvents and chlorine-containing materials so as to reduce the corrosion of equipment and contamination of environment.
Currently, there are three main methods to synthesize diphenyl carbonate: the phosgene method, the ester exchange method and the method of oxidative carbonylation of phenol. The phosgene method is the earliest method and also the main method for producing diphenyl carbonate in the past. However, this method is complex in its process, high in cost and poor in quality, and the phosgene is very toxic and contaminates the environment severely. Thus, in foreign countries, the phosgene method has been weeded out. The ester exchange method uses dimethyl carbonate (U.S. Pat. No. 4,410,464) or dimethyl oxalate (JP 08-325207) so as to avoid the use of phosgene; however, its equilibrium conversion is low, the recovery of homogenous catalysts is difficult and it requires special reaction equipment. The method of oxidative carbonylation of phenol uses carbon monoxide, oxygen, phenol directly to synthesize diphenyl carbonate in one step. This method has a simple process, uses cheap raw materials and does not contaminate the environment, which is an attractive process route and is well worth further developing and researching.
U.S. Pat. No. 4,096,168 of General Electric Company discloses a diaryl carbonate process, comprising phenol, carbon monoxide, a base, and a Group VIIIB metal compound having an oxidation state greater than zero used as the catalyst, wherein said base is a sterically hindered amine. U.S. Pat. No. 4,096,169 also discloses that this reaction system can be carried out in the absence of any solvent, when phenols play a dual role of reactant and solvent, or in the presence of a solvent, and a suitable solvent can be methylene chloride, toluene, diphenylether, chlorobenzene, o-dichlorobenzene, etc. In addition, the base in the catalytic system can be an organic or inorganic base, such as alkali metals or alkaline earth metals and their hydroxides, quaternary ammonium and phosphonium, primary, secondary or tertiary amines, etc. Because this catalytic system cannot be re-oxidized to its original oxidation number after the oxidation state of the Group VIIIB metal having an oxidation state greater than zero reduces to zero as the reaction carries out, the reaction terminates. Also, this process uses organic solvents, which may cause contamination.
U.S. Pat. No. 4,349,485 discloses a diaryl carbonate process, comprising phenol, carbon monoxide, a base and a Group VIIIB metal, and further comprising an oxidant and a redox co-catalyst of manganese tetradentates. The manganese tetradentates are of the formula (L)xMn wherein L is bis(β-diketone) (C14-20H22-34O4), the oxidant is air, and a molecular sieve and tetrabutylammonium bromide are used as the drying agent and the phase transfer agent, respectively. The reaction time of this process is 80 hr and the convertibility of phenol is about 50%, which reactivity is too low.
U.S. Pat. No. 5,132,447 discloses the use of a homogenous catalytic system of palladium (II) acetate/cobalt (II) diacetate/tetra-n-butylammonium bromide, with benzoquinone added, to increase the yield of diphenyl carbonate from 15.2% to 26.1% at a high pressure (the maximum pressure is up to 2050 psi). U.S. Pat. No. 5,284,964 discloses the finding that the yield of diphenyl carbonate can achieve 45% by using palladium (II) acetate as the main catalyst, cobalt di-(salicylal)-3,3′-diamino-N-methyldipropylamine (CoSMDPT) as the inorganic cocatalyst, tetraalkylammonium bromide or hexaalkylguanidinium bromide as the source of bromides in the presence of the organic cocatalyst of terpyridine and introducing carbon monoxide and oxygen in a fixed ratio at a high pressure (the maximum pressure is up to 1600 psi). In order to achieve commercially acceptable reaction rate and selectivity, this process must be carried out at a high pressure. However, under the condition that the total reaction pressure increases continuously, the equipment investment cost will be increased greatly when commercialization.
EP 350,700 uses a cobalt salt as the inorganic cocatalyst with a quinine or hydroquinone added as the electron transfer catalyst. However, in this process, the removal of electron transfer catalyst is extremely costly. The two OH groups provided by hydroquinone will also cause the phenol to form byproducts of carbonates, and the removal of such byproducts is costly. Also, the electron transfer catalyst cannot be recovered for reuse and the formation of byproducts lowers the selectivity. Thus the economic burden increases.
The process disclosed by JP 04-257546 is carried out in a distillation tower, which uses noble metals and quaternary salts as the catalytic system and removes reaction water by distillation. Due to the equipment problem, the holdup time is very short so that the space-time yield (STY) is very low and is merely 17.8 g/1 h. Also, a large amount of halogen ions exist in the catalytic system used by this process, which will cause corrosion.
U.S. Pat. No. 5,498,742 uses palladium bromide/tetrabutylammonium bromide/manganese(II) acetylacetonate/sodium phenolate as the catalytic system. However, the catalyst should be activated with a large amount of carbon monoxide first, which is not economically beneficial.